This manual is for Plzip (version 1.10, 24 January 2022).
Copyright © 2009-2022 Antonio Diaz Diaz.
This manual is free documentation: you have unlimited permission to copy, distribute, and modify it.
Plzip is a massively parallel (multi-threaded) implementation of lzip, fully compatible with lzip 1.4 or newer. Plzip uses the compression library lzlib.
Lzip is a lossless data compressor with a user interface similar to the one of gzip or bzip2. Lzip uses a simplified form of the 'Lempel-Ziv-Markov chain-Algorithm' (LZMA) stream format and provides a 3 factor integrity checking to maximize interoperability and optimize safety. Lzip can compress about as fast as gzip (lzip -0) or compress most files more than bzip2 (lzip -9). Decompression speed is intermediate between gzip and bzip2. Lzip is better than gzip and bzip2 from a data recovery perspective. Lzip has been designed, written, and tested with great care to replace gzip and bzip2 as the standard general-purpose compressed format for unix-like systems.
Plzip can compress/decompress large files on multiprocessor machines much faster than lzip, at the cost of a slightly reduced compression ratio (0.4 to 2 percent larger compressed files). Note that the number of usable threads is limited by file size; on files larger than a few GB plzip can use hundreds of processors, but on files of only a few MB plzip is no faster than lzip. See Minimum file sizes.
For creation and manipulation of compressed tar archives tarlz can be more efficient than using tar and plzip because tarlz is able to keep the alignment between tar members and lzip members.
The lzip file format is designed for data sharing and long-term archiving, taking into account both data integrity and decoder availability:
A nice feature of the lzip format is that a corrupt byte is easier to repair the nearer it is from the beginning of the file. Therefore, with the help of lziprecover, losing an entire archive just because of a corrupt byte near the beginning is a thing of the past.
Plzip uses the same well-defined exit status values used by lzip, which makes it safer than compressors returning ambiguous warning values (like gzip) when it is used as a back end for other programs like tar or zutils.
Plzip will automatically use for each file the largest dictionary size that does not exceed neither the file size nor the limit given. Keep in mind that the decompression memory requirement is affected at compression time by the choice of dictionary size limit. See Memory requirements.
When compressing, plzip replaces every file given in the command line with a compressed version of itself, with the name "original_name.lz". When decompressing, plzip attempts to guess the name for the decompressed file from that of the compressed file as follows:
(De)compressing a file is much like copying or moving it. Therefore plzip preserves the access and modification dates, permissions, and, when possible, ownership of the file just as 'cp -p' does. (If the user ID or the group ID can't be duplicated, the file permission bits S_ISUID and S_ISGID are cleared).
Plzip is able to read from some types of non-regular files if either the option '-c' or the option '-o' is specified.
Plzip will refuse to read compressed data from a terminal or write compressed data to a terminal, as this would be entirely incomprehensible and might leave the terminal in an abnormal state.
Plzip will correctly decompress a file which is the concatenation of two or more compressed files. The result is the concatenation of the corresponding decompressed files. Integrity testing of concatenated compressed files is also supported.
The output of plzip looks like this:
plzip -v foo foo: 6.676:1, 14.98% ratio, 85.02% saved, 450560 in, 67493 out. plzip -tvvv foo.lz foo.lz: 6.676:1, 14.98% ratio, 85.02% saved. 450560 out, 67493 in. ok
The meaning of each field is as follows:
When decompressing or testing at verbosity level 4 (-vvvv), the dictionary size used to compress the file is also shown.
LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may never have been compressed. Decompressed is used to refer to data which have undergone the process of decompression.
The format for running plzip is:
plzip [options] [files]
If no file names are specified, plzip compresses (or decompresses) from standard input to standard output. A hyphen '-' used as a file argument means standard input. It can be mixed with other files and is read just once, the first time it appears in the command line.
plzip supports the following options:
If any file is damaged, does not exist, can't be opened, or is not regular,
the final exit status will be > 0. '-lq' can be used to verify
quickly (without decompressing) the structural integrity of the files
specified. (Use '--test' to verify the data integrity). '-alq'
additionally verifies that none of the files specified contain trailing data.
Plzip starts the number of threads required by each file without exceeding
the value specified. Note that the number of usable threads is limited to
ceil( file_size / data_size ) during compression (see Minimum file sizes), and to the number of members in the input during decompression. You
can find the number of members in a lzip file by running
'plzip -lv file.lz'.
In order to keep backward compatibility with plzip versions prior to 1.9,
when compressing from standard input and no other file names are given, the
extension '.lz' is appended to file unless it already ends in
'.lz' or '.tlz'. This feature will be removed in a future version
of plzip. Meanwhile, redirection may be used instead of '-o' to write
the compressed output to a file without the extension '.lz' in its
name: 'plzip < file > foo'.
For maximum compression you should use a dictionary size limit as large
as possible, but keep in mind that the decompression memory requirement
is affected at compression time by the choice of dictionary size limit.
-0 .. -9
The bidimensional parameter space of LZMA can't be mapped to a linear scale optimal for all files. If your files are large, very repetitive, etc, you may need to use the options '--dictionary-size' and '--match-length' directly to achieve optimal performance.
If several compression levels or '-s' or '-m' options are given, the last setting is used. For example '-9 -s64MiB' is equivalent to '-s64MiB -m273'
|Level||Dictionary size (-s)||Match length limit (-m)
|-0||64 KiB||16 bytes
|-1||1 MiB||5 bytes
|-2||1.5 MiB||6 bytes
|-3||2 MiB||8 bytes
|-4||3 MiB||12 bytes
|-5||4 MiB||20 bytes
|-6||8 MiB||36 bytes
|-7||16 MiB||68 bytes
|-8||24 MiB||132 bytes
|-9||32 MiB||273 bytes
Numbers given as arguments to options may be followed by a multiplier and an optional 'B' for "byte".
Table of SI and binary prefixes (unit multipliers):
|k||kilobyte (10^3 = 1000)|||||Ki||kibibyte (2^10 = 1024)
|M||megabyte (10^6)|||||Mi||mebibyte (2^20)
|G||gigabyte (10^9)|||||Gi||gibibyte (2^30)
|T||terabyte (10^12)|||||Ti||tebibyte (2^40)
|P||petabyte (10^15)|||||Pi||pebibyte (2^50)
|E||exabyte (10^18)|||||Ei||exbibyte (2^60)
|Z||zettabyte (10^21)|||||Zi||zebibyte (2^70)
|Y||yottabyte (10^24)|||||Yi||yobibyte (2^80)
Exit status: 0 for a normal exit, 1 for environmental problems (file not found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or invalid input file, 3 for an internal consistency error (e.g., bug) which caused plzip to panic.
When compressing, plzip divides the input file into chunks and compresses as many chunks simultaneously as worker threads are chosen, creating a multimember compressed file. Each chunk is compressed in-place (using the same buffer for input and output), reducing the amount of RAM required.
When decompressing, plzip decompresses as many members simultaneously as worker threads are chosen. Files that were compressed with lzip will not be decompressed faster than using lzip (unless the option '-b' was used) because lzip usually produces single-member files, which can't be decompressed in parallel.
For each input file, a splitter thread and several worker threads are created, acting the main thread as muxer (multiplexer) thread. A "packet courier" takes care of data transfers among threads and limits the maximum number of data blocks (packets) being processed simultaneously.
The splitter reads data blocks from the input file, and distributes them to the workers. The workers (de)compress the blocks received from the splitter. The muxer collects processed packets from the workers, and writes them to the output file.
.------------. ,-->| worker 0 |--, | `------------' | .-------. .----------. | .------------. | .-------. .--------. | input |-->| splitter |-+-->| worker 1 |--+-->| muxer |-->| output | | file | `----------' | `------------' | `-------' | file | `-------' | ... | `--------' | .------------. | `-->| worker N-1 |--' `------------'
When decompressing from a regular file, the splitter is removed and the workers read directly from the input file. If the output file is also a regular file, the muxer is also removed and the workers write directly to the output file. With these optimizations, the use of RAM is greatly reduced and the decompression speed of large files with many members is only limited by the number of processors available and by I/O speed.
The amount of memory required per worker thread for decompression or testing is approximately the following:
The amount of memory required per worker thread for compression is approximately the following:
The following table shows the memory required per thread for compression at a given level, using the default data size for each level:
When compressing, plzip divides the input file into chunks and compresses as many chunks simultaneously as worker threads are chosen, creating a multimember compressed file.
For this to work as expected (and roughly multiply the compression speed by the number of available processors), the uncompressed file must be at least as large as the number of worker threads times the chunk size (see --data-size). Else some processors will not get any data to compress, and compression will be proportionally slower. The maximum speed increase achievable on a given file is limited by the ratio (file_size / data_size). For example, a tarball the size of gcc or linux will scale up to 10 or 14 processors at level -9.
The following table shows the minimum uncompressed file size needed for full use of N processors at a given compression level, using the default data size for each level:
|-0||2 MiB||4 MiB||8 MiB||16 MiB||64 MiB||256 MiB
|-1||4 MiB||8 MiB||16 MiB||32 MiB||128 MiB||512 MiB
|-2||6 MiB||12 MiB||24 MiB||48 MiB||192 MiB||768 MiB
|-3||8 MiB||16 MiB||32 MiB||64 MiB||256 MiB||1 GiB
|-4||12 MiB||24 MiB||48 MiB||96 MiB||384 MiB||1.5 GiB
|-5||16 MiB||32 MiB||64 MiB||128 MiB||512 MiB||2 GiB
|-6||32 MiB||64 MiB||128 MiB||256 MiB||1 GiB||4 GiB
|-7||64 MiB||128 MiB||256 MiB||512 MiB||2 GiB||8 GiB
|-8||96 MiB||192 MiB||384 MiB||768 MiB||3 GiB||12 GiB
|-9||128 MiB||256 MiB||512 MiB||1 GiB||4 GiB||16 GiB
Perfection is reached, not when there is no longer anything to add, but
when there is no longer anything to take away.
-- Antoine de Saint-Exupery
In the diagram below, a box like this:
+---+ | | <-- the vertical bars might be missing +---+
represents one byte; a box like this:
+==============+ | | +==============+
represents a variable number of bytes.
A lzip file consists of a series of independent "members" (compressed data sets). The members simply appear one after another in the file, with no additional information before, between, or after them. Each member can encode in compressed form up to 16 EiB - 1 byte of uncompressed data. The size of a multimember file is unlimited.
Each member has the following structure:
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID string | VN | DS | LZMA stream | CRC32 | Data size | Member size | +--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All multibyte values are stored in little endian order.
Sometimes extra data are found appended to a lzip file after the last member. Such trailing data may be:
Trailing data are in no way part of the lzip file format, but tools reading lzip files are expected to behave as correctly and usefully as possible in the presence of trailing data.
Trailing data can be safely ignored in most cases. In some cases, like that of user-added data, they are expected to be ignored. In those cases where a file containing trailing data must be rejected, the option '--trailing-error' can be used. See --trailing-error.
WARNING! Even if plzip is bug-free, other causes may result in a corrupt compressed file (bugs in the system libraries, memory errors, etc). Therefore, if the data you are going to compress are important, give the option '--keep' to plzip and don't remove the original file until you verify the compressed file with a command like 'plzip -cd file.lz | cmp file -'. Most RAM errors happening during compression can only be detected by comparing the compressed file with the original because the corruption happens before plzip compresses the RAM contents, resulting in a valid compressed file containing wrong data.
Example 1: Extract all the files from archive 'foo.tar.lz'.
tar -xf foo.tar.lz or plzip -cd foo.tar.lz | tar -xf -
Example 2: Replace a regular file with its compressed version 'file.lz' and show the compression ratio.
plzip -v file
Example 3: Like example 2 but the created 'file.lz' has a block size of 1 MiB. The compression ratio is not shown.
plzip -B 1MiB file
Example 4: Restore a regular file from its compressed version 'file.lz'. If the operation is successful, 'file.lz' is removed.
plzip -d file.lz
Example 5: Verify the integrity of the compressed file 'file.lz' and show status.
plzip -tv file.lz
Example 6: The right way of concatenating the decompressed output of two or more compressed files. See Trailing data.
Don't do this cat file1.lz file2.lz file3.lz | plzip -d - Do this instead plzip -cd file1.lz file2.lz file3.lz
Example 7: Decompress 'file.lz' partially until 10 KiB of decompressed data are produced.
plzip -cd file.lz | dd bs=1024 count=10
Example 8: Decompress 'file.lz' partially from decompressed byte at offset 10000 to decompressed byte at offset 14999 (5000 bytes are produced).
plzip -cd file.lz | dd bs=1000 skip=10 count=5
Example 9: Compress a whole device in /dev/sdc and send the output to 'file.lz'.
plzip -c /dev/sdc > file.lz or plzip /dev/sdc -o file.lz
There are probably bugs in plzip. There are certainly errors and omissions in this manual. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer.
If you find a bug in plzip, please send electronic mail to email@example.com. Include the version number, which you can find by running 'plzip --version' and 'plzip -v --check-lib'.